write an essay describing how life changes through natural selection

ENCYCLOPEDIC ENTRY

Natural selection.

Natural selection is the process through which species adapt to their environments. It is the engine that drives evolution.

On the Origin of Species

English naturalist Charles Darwin wrote the definitive book outlining his idea of natural selection, On the Origin of Species. The book chronicled his studies in South America and Pacific islands. Published in 1859, the book became a best seller.

Photograph by Ian Forsyth via Getty Images

English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species .

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits — traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection , favorable traits are transmitted through generations .

Natural selection can lead to speciation , where one species gives rise to a new and distinctly different species . It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.

Darwin chose the name natural selection to contrast with “artificial selection,” or selective breeding that is controlled by humans. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.

Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.

Darwin did not know that genes existed, but he could see that many traits are heritable—passed from parents to offspring.

Mutations are changes in the structure of the molecules that make up genes , called DNA . The mutation of genes is an important source of genetic variation within a population. Mutations can be random (for example, when replicating cells make an error while copying DNA ), or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.

Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells (eggs and sperm), they can be passed on to offspring.

If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection . Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species , a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species .

Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.

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Natural Selection: Darwin’s Theory of Evolution Essay

According to Charles Darwin, natural selection is a process whereby the survival of different living organisms depends on their gradual adaptation to certain environments over many generations.

It is commonly known by the phrase, ”survival of the fittest”, which means that only the species that have well adapted to their environment, is well suited to survive in that habitat. The theory of natural selection by Charles Darwin also states that, variations in size, shape, strength, and color do occur naturally in all living things.

These natural variations, called mutations through evolution, affect which living organisms will survive to live long enough to reproduce. For instance, animals with traits or qualities that are well suited to their environment, such as long legs in wading birds, are more likely to survive long enough to breed than others of their species.

When these animals breed, they may pass on the favorable traits to their offspring through their genes, while those with unfavorable traits are most likely to die without reproducing. As more and more organisms in a particular species inherit a favorable trait, the gene becomes more common in the population, and so the species changes.

Reactions to Charles Darwin’s Theory

Creation theory.

Darwin’s theory of evolution by natural selection encountered a sharp reaction especially at Evangelical Protestantism, since it greatly undermined the story of creation by God and current defenses of the faith at two critical points. By implication, it questioned the audacity of accuracy of the Bible, which had been his most important exhibit in demonstrations of “evidences” for Christianity. Secondly, Darwinism, as the theory came to be known, also totally reversed the perceptions of the relation of science to the Christian faith.

In the mid-nineteenth century, American Christian apologists rested their case heavily on the argument through scientific revolution, by uncovering some of the marvels of God’s intricate and awesome design of the universe. They argued that it was inconsistent to rationally believe that such a complex and orderly system could lack an intelligent designer.

In addition, the Protestant reactions to Darwinism did vary considerably, they argued that if Darwinism had to do simply with biological development, the process it posited could only be subsumed under God’s providence, and they suggested that evolution was a way of God doing things.

Lamarck’s theory

During this period, the American scientific arena was dominated by a formidable number of scientists who did not find the natural-selection hypothesis adequate enough. A few naturalists endowed with much flexibility of mind also doubted the immutability of species.

Majority of the scientists held allegiance to Jean Baptiste Lamarck’s theory that evolution was evident as organisms adapted to environments to meet their biological needs out of resources in such environments and the instruments that they effectively employed would develop further, while the inefficient ones atrophied.

These features according to Lamarck’s theory were inheritable, and the species were directed towards a goal whose progress seemed inevitable. So, with the perception of Lamarckism, progressive religionists quickly adapted and saw evolution as God’s way of doing things.

One example given by Lamarck to support his theory was that, ancestors of modern giraffe were deer like animals with short neck and small forelimbs, and so for it to survive, a giraffe had to stretch their necks so as to feed on the tall trees which had remained from a period of drought.

Due to the continuous stretching, the length of the neck and forelimbs increased, therefore making all acquired characters inherited.

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Evolution by Natural Selection: Examples and Effects of Adaptation

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Several hundred million years ago, there were no vertebrate animals on land. The only vertebrate species in the world were fish, all of which lived underwater. Competition for food was intense.

This is where one of the best natural selection examples came into play. Some species of fish that lived near the coast developed a strange mutatio­n: the ability to push themselves along in the mud and sand on the shore with their fins.

This gave them access to food sources that no other fish could reach. The advantage gave them greater reproductive success, so the mutation was passed along.

What Is Natural Selection?

Understanding evolution, population pressure, the superorganism vs. the selfish gene, natural selection examples.

Natural selection is the engine that drives evolution . The individual organisms with the variation best suited to survive in their particular circumstances have a greater chance of passing that trait on to the next generation.

But plants and animals interact in very complex ways with other organisms and their environment. These factors work together to produce the amazingly diverse range of life forms present on Earth .

By understanding natural selection, we can learn why some plants produce cyanide, why rabbits produce so many offspring, how animals first emerged from the ocean to live on land, and how some mammals eventually went back again. We can even learn about microscopic life, such as bacteria and viruses , or figure out how humans became humans.

Charles Darwin coined the term "natural selection." You'll typically hear it alongside the often misunderstood evolutionary catchphrase "survival of the fittest."

But survival of the fittest isn't necessarily the bloody, tooth-and-claw battle for survival we tend to make it out to be (although sometimes it is).

Rather, natural selection occurs as species change to adapt to life: how efficient a tree is at dispersing seeds; a fish's ability to find a safe spawning ground before laying her eggs; the skill with which a bird retrieves seeds from the deep, fragrant cup of a flower; a bacterium's resistance to antibiotics.

With a little help from Darwin himself, we're going to learn how natural selection explains the astonishing complexity and diversity of life on planet Earth.

Evolution is the result of the tendency for some organisms to have better reproductive success than others — natural selection.

Inherited Traits

It's important to remember that differences between individuals, even individuals from different generations, don't constitute evolution. Those are just variations of traits.

Traits are characteristics that are inheritable — they can be passed down from one generation to the next. Not all traits are physical — the ability to tolerate close contact with humans is a trait that evolved in dogs . Here's an example that helps explain these concepts:

Basketball players are generally tall, while jockeys are generally short. This is a genetic variation on the trait of height. Tall parents tend to have tall children, so we can see that the trait is inheritable.

Now imagine that some conditions arise that make it more likely for jockeys to reproduce successfully than basketball players. Jockeys have children more frequently, and these children tend to be short. Basketball players have fewer children, so there are fewer tall people. After a few generations, the average height of humans decreases. Humans have evolved to be shorter.

Allele Frequency

Evolution is all about change over time, but what is the mechanism that causes these changes? Every living thing has everything about its construction encoded in a special chemical structure called DNA .

Within the DNA are chemical sequences that define a certain trait or set of traits. These sequences are known as genes. The part of each gene that results in the varying expression of traits is called an allele.

Because a trait is an expression of an allele, the tendency of a certain trait to show up in a population is referred to as allele frequency. In essence, evolution is a change in allele frequencies over the course of several generations.

Different alleles (and thus different traits) are created in three ways:

• Mutations are random changes that occur in genes. They're relatively rare, but over thousands of generations, they can add up to very profound changes. Mutations can introduce traits that are completely new and have never appeared in that species before.
• Sexual reproduction mixes the genes of each parent by splitting, breaking and blending chromosomes (the strands that contain DNA) during the creation of each sperm and egg. When the sperm and the egg combine, some genes from the male parent and some genes from the female parent are blended randomly, creating a unique mix of alleles in their offspring. ­
• Bacteria, which don't reproduce sexually, can absorb bits of DNA they encounter and incorporate it into their own genetic code through various methods of genetic recombination .

Sexual reproduction itself is a product of natural selection — organisms that blend genes in this way gain access to a greater variety of traits, making them more likely to find the right traits for survival.

What's a Population?

A population is a defined group of organisms. In terms of evolutionary science, a population usually refers to a group of organisms that have reproductive access to each other. For example, zebras that live on the plains of Africa are a population.

If other wild zebras lived in South America (none do, but let's pretend they do for the sake of the example), they would represent a different population because they're too far away to mate with the African zebras. Lions that live on the plains of Africa are a different population as well, because lions and zebras are biologically unable to mate with each other.

Fitness is the key to natural selection. We're not talking about how many reps a sea otter can burn through at the gym; biological fitness is an organism's ability to successfully survive long enough to produce offspring.

Beyond that, it also reflects an organism's ability to reproduce well. It isn't enough for a tree to create a bunch of seeds. Those seeds need the ability to end up in fertile soil with enough resources to sprout and grow.

Fitness and natural selection were first explained in detail by Charles Darwin, who observed wildlife around the world, took copious notes, then sought to understand what he had seen. Natural selection is probably best explained in his words, taken from his landmark work "On the Origin of Species."

• Organisms show variation of traits. "The many slight differences which appear in the offsprin­g of the same parents may be called individual differences. No one supposes that all the individuals of the same species are cast in the same actual mould."
• More organisms are born than could ever possibly be supported by the planet's resources. "Every being … must suffer destruction at some period of its life, otherwise, on the principle of geometrical increase, its numbers would quickly become so … great that no country could support the product."
• Therefore, all organisms must struggle to live. "As more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life."
• Some advantageous traits aid in the struggle to survive and reproduce. "Can we doubt … that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating?"
• Organisms that have those helpful traits are more likely to successfully reproduce and pass the traits on to the next generation. "The slightest differences may turn the nicely balanced scale in the struggle for life, and so be preserved."
• Successful variations accumulate over the generations as the organisms are exposed to population pressure. "Natural Selection acts exclusively by the preservation and accumulation of variations which are beneficial under the conditions to which each creature is exposed. The ultimate result is that each creature tends to become more and more improved in relation to its conditions."

The process of natural selection can be sped up immensely by strong population pressures. Population pressure is a circumstance that makes it harder for organisms to survive. There's always some kind of population pressure, but events like floods , droughts or new predators can increase it.

Under high pressure, more members of a population will die before reproducing. This means that only those individuals with traits that allow them to deal with the new pressure will survive and pass along their alleles to the next generation. This can result in drastic changes to allele frequencies within one or two generations.

Example of Population Pressure

Imagine a giraffe population with individuals that range in height from 10 feet to 20 feet tall. One day, a brush fire sweeps through and destroys all the vegetation below 15 feet. Only the giraffes taller than 15 feet can reach the higher leaves to eat.

Giraffes below that height are unable to find any food at all. Most of them starve before they can reproduce. In the next generation, very few short giraffes are born. The population's average height goes up by several feet.

Population Bottleneck

There are other ways to quickly and drastically affect allele frequency. One way is a population bottleneck.

In a large population, alleles are evenly distributed across the population. If some event, such as a disease or a drought, wipes out a large percentage of the population, the remaining individuals may have an allele frequency very different from the larger population.

By pure chance, they may have a high concentration of alleles that were relatively rare before. As these individuals reproduce, the formerly rare traits become the average for the population.

Founder Effect

The founder effect can also bring about rapid evolution. This occurs when a small number of individuals migrate to a new location, "founding" a new population that no longer mates with the old population.

Just as with a population bottleneck, these individuals may have unusual allele frequencies, leading subsequent generations to have very different traits from the original population that the founders migrated from.

The difference between slow, gradual changes over many generations (gradualism) and rapid changes under high population pressure interspersed with long periods of evolutionary stability (punctuated equilibrium) is an ongoing debate in evolutionary science.

Evolutionary Stability

So far, we've looked at natural selection as an agent of change. When we look around the world, however, we see many animals that have remained relatively unchanged for tens of thousands of years — in some cases, even millions of years. Sharks are one example.

It turns out that natural selection is also an agent of stability.

Sometimes an organism reaches a state of evolution in which its traits are very well-suited to its environment. When nothing happens to exert strong population pressure on that population, natural selection favors the allele frequency already present.

When mutations cause new traits, natural selection weeds these traits out because they're not as efficient as the others.

Evolutionary biologist Richard Dawkins wrote a book called "The Selfish Gene" in the 1970s. Dawkins' book reframed evolution by pointing out that natural selection favors the passing on of genes, not the organism itself.

Once an organism has successfully reproduced, natural selection doesn't care what happens after. This explains why certain strange traits continue to exist — traits that seem to cause harm to the organism but benefit the genes.

In some spider species, the female eats the male after mating. As far as natural selection is concerned, a male spider that dies 30 seconds after mating is just as successful as one that lives a full, rich life.

Altruism and Kinship

Since the publication of "The Selfish Gene," most biologists agree that Dawkins' ideas explain a great deal about natural selection, but they don't answer everything. One of the main sticking points is altruism.

Why do people (and many animal species) do good things for others, even when it offers no direct benefit to themselves? Research has shown that this behavior is instinctive and appears without cultural training in human infants [source: Barragan et al. ]. It also appears in some primate species. Why would natural selection favor an instinct to help others?

One theory revolves around kinship. People who are related to you share many of your genes. Helping them could help ensure that some of your genes are passed down. Imagine two families of early humans, both competing for the same food sources.

One family has alleles for altruism — they help each other hunt and share food. The other family doesn't — they hunt separately, and each human only eats whatever he can catch. The cooperative group is more likely to achieve reproductive success, passing along the alleles for altruism.

Superorganism

Biologists are also exploring a concept known as the superorganism. It's basically an organism made out of many smaller organisms. The model superorganism is the insect colony.

In an ant colony, only the queen and a few males will ever pass their genes to the next generation. Thousands of other ants spend their entire lives as workers or drones with absolutely no chance of passing on their genes directly. Yet they work to contribute to the success of the colony.

In terms of the "selfish gene," this doesn't make a whole lot of sense. But if you look at an insect colony as a single organism made up of many small parts (the ants), it does. Each ant works to ensure the reproductive success of the colony as a whole. Some scientists think the superorganism concept can be used to explain some aspects of human evolution [source: Keim ].

Vestigial and Atavistic Traits

All organisms carry traits that no longer confer any real benefit to them in terms of natural selection. If the trait doesn't harm the organism, then natural selection won't weed it out, so these traits stick around for generations. The result: organs and behaviors that no longer serve their original purpose. These traits are called vestigial.

There are many examples in the human body alone. The tailbone is the remnant of an ancestor's tail, and the ability to wiggle your ears is leftover from an earlier primate that was able to move their ears around to pinpoint sounds.

Plants have vestigial traits as well. Many plants that once reproduced sexually (requiring pollination by insects) evolved the ability to reproduce asexually. They no longer need insects to pollinate them, but they still produce flowers, which were originally needed to entice insects to visit the plant.

Sometimes, a mutation causes a vestigial trait to express itself more fully. This is known as an atavism . Humans are sometimes born with small tails. It's fairly common to find whales with hind legs. Sometimes snakes have the equivalent of toenails, even though they don't have toes. Or feet.

We usually think of evolution as something we don't see happening right before our eyes, instead looking at fossils to find evidence of it happening in the past. In fact, evolution under intense population pressure happens so fast that we've seen it occur within the span of a human lifetime.

Elephant Tusks

African elephants typically have large tusks. The ivory in the tusks is highly valued by some people, so hunters have hunted and killed elephants to tear out their tusks and sell them (usually illegally) for decades.

Some African elephants have a rare trait: They never develop tusks at all. In 1930, about 1 percent of all elephants had no tusks. The ivory hunters didn't bother killing them because there was no ivory to recover. Meanwhile, elephants with tusks were killed off by the hundreds, many of them before they ever had a chance to reproduce.

The alleles for "no tusks" were passed along over just a few generations. The result: As many as half of the female elephants in some modern populations have no tusks [source: BBC News , New York Times ]. Unfortunately, this isn't really a happy ending for the elephants, since their tusks are used for digging and defense.

Pest Resistance

The bollworm, a pest that eats and damages cotton crops, has shown that natural selection can act even faster than scientists can genetically engineer something. Some cotton crops have been genetically modified to produce a toxin that's harmful to most bollworms.

A small number of bollworms had a mutation that gave them immunity to the toxin. They ate the cotton and lived, while all nonimmune bollworms died. The intense population pressure has produced broad immunity to the toxin in the entire species within the span of just a few years [source: EurekAlert ].

Clover and Cyanide

Some species of clover developed a mutation that caused the poison cyanide to form in the plant's cells . This gave the clover a bitter taste, making it less likely to be eaten. However, when the temperature drops below freezing, some cells rupture, releasing the cyanide into the plant's tissues and killing the plant.

In warm climates, natural selection acted in favor of the cyanide-producing clover, but where the winters are cold, non-cyanide clover was favored. Each kind exists almost exclusively in each climate area [source: Purves].

Natural Selection in Humans

What about humans? Are we subject to natural selection as well? It's certain that we were — humans only became humans because an assortment of traits (larger brains, walking upright) conferred advantages to those primates that developed them. But we're capable of influencing the distribution of our genes directly.

We can use birth control, so that those of who are "fittest" in terms of natural selection might not pass on our genes at all. We use medicine and science to allow many people to live (and reproduce ) who otherwise wouldn't likely survive past childhood. Much like domesticated animals, which we breed to specifically favor certain traits, humans are influenced by a sort of unnatural selection.

However, we're still evolving. Some humans have more reproductive success than others, and the factors that affect that equation have added a layer of human complexity on top of the already complicated interactions of the animal world.

In other words, we don't really know what we're going to evolve into. Change is inevitable, but remember that natural selection doesn't care about making "better" humans, just more of us.

Lots More Information

Related howstuffworks articles.

• How Evolution Works
• How Atavisms Work
• Why do humans walk on two legs?
• How Animal Migration Works
• How Human Migration Works
• How the Scientific Method Works
• How Creationism Works
• How Intelligent Design Works
• How DNA Works
• How Human Reproduction Works
• BBC. "Africa Elephants 'ditch tusks' to survive." Sept. 25, 1998.http://news.bbc.co.uk/1/hi/world/africa/180301.stm
• CBC News. "Infants show early signs of altruism." March 2, 2006. http://www.cbc.ca/health/story/2006/03/02/altruism060302.html
• Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. 1859.
• Dawkins, Richard. The Selfish Gene. Oxford University Press, USA; 3 edition. May 25, 2006.
• Keim, Brandon. "A Brief History of the Superorganism, Part One." Wired, July 11, 2007. http://blog.wired.com/wiredscience/2007/07/a-brief-history.html
• Purves, William K., Sadava, David, Orians, Gordon H., and Heller, H. Craig. Life: The Science of Biology. Sinauer Associates and W. H. Freeman. December 5, 2003.
• University of Arizona College of Agriculture and Life Sciences. "First documented case of pest resistance to biotech cotton." http://www.eurekalert.org/pub_releases/2008-02/uoa-fdc020508.php
• Winning, Bob. "Recombination in Bacteria." http://www.emunix.emich.edu/~rwinning/genetics/bactrec.htm

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24 Charles Darwin and Natural Selection

In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle , including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 1).

The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch’s varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.

Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection , or “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change.

For example, Darwin observed a population of giant tortoises in the Galápagos Archipelago to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading economist Thomas Malthus’ essay that explained this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment. It is the only mechanism known for adaptive evolution.

In 1858, Darwin and Wallace (Figure 2) presented papers at the Linnean Society in London that discussed the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for evolution by natural selection.

It is difficult and time-consuming to document and present examples of evolution by natural selection. The Galápagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important evidence of natural selection. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds of which the large-billed birds ate; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds were able to survive and reproduce. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

CAREER CONNECTION

Field biologist.

Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 3).

One objective of many field biologists includes discovering new, unrecorded species. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When discovered, researchers can use these important species as evidence for environmental regulations and laws.

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Understanding Evolution

Your one-stop source for information on evolution

The History of Evolutionary Thought

Natural selection: charles darwin & alfred russel wallace.

Pre-Darwinian ideas about evolution

It was Darwin’s genius both to show how all this evidence favored the evolution of species from a common ancestor and to offer a plausible mechanism by which life might evolve. Lamarck and others had promoted evolutionary theories, but in order to explain just how life changed, they depended on speculation. Typically, they claimed that evolution was guided by some long-term trend. Lamarck, for example, thought that life strove over time to rise from simple single-celled forms to complex ones. Many German biologists conceived of life evolving according to predetermined rules, in the same way an embryo develops in the womb. But in the mid-1800s, Darwin and the British biologist Alfred Russel Wallace independently conceived of a natural, even observable, way for life to change: a process Darwin called  natural selection.

The pressure of population growth

Interestingly, Darwin and Wallace found their inspiration in economics. An English parson named  Thomas Malthus  published a book in 1797 called  Essay on the Principle of Population  in which he warned his fellow Englishmen that most policies designed to help the poor were doomed because of the relentless pressure of population growth. A nation could easily double its population in a few decades, leading to famine and misery for all.

When Darwin and Wallace read Malthus, it occurred to both of them that animals and plants should also be experiencing the same population pressure. It should take very little time for the world to be knee-deep in beetles or earthworms. But the world is not overrun with them, or any other species, because they cannot reproduce to their full potential. Many die before they become adults. They are vulnerable to droughts and cold winters and other environmental assaults. And their food supply, like that of a nation, is not infinite. Individuals must compete, albeit unconsciously, for what little food there is.

Selection of traits

In this struggle for existence, survival and reproduction do not come down to pure chance. Darwin and Wallace both realized that if an animal has some trait that helps it to withstand the elements or to breed more successfully, it may leave more offspring behind than others. On average, the trait will become more common in the following generation, and the generation after that.

As Darwin wrestled with  natural selection  he spent a great deal of time with pigeon breeders, learning their methods. He found their work to be an analogy for evolution. A pigeon breeder selected individual birds to reproduce in order to produce a neck ruffle. Similarly, nature unconsciously “selects” individuals better suited to surviving their local conditions. Given enough time, Darwin and Wallace argued, natural selection might produce new types of body parts, from wings to eyes.

Darwin and Wallace develop similar theory

Darwin began formulating his theory of natural selection in the late 1830s but he went on working quietly on it for twenty years. He wanted to amass a wealth of evidence before publicly presenting his idea. During those years he corresponded briefly with Wallace (right), who was exploring the wildlife of South America and Asia. Wallace supplied Darwin with birds for his studies and decided to seek Darwin’s help in publishing his own ideas on evolution. He sent Darwin his theory in 1858, which, to Darwin’s shock, nearly replicated Darwin’s own.

Charles Lyell  and Joseph Dalton Hooker arranged for both Darwin’s and Wallace’s theories to be presented to a meeting of the Linnaean Society in 1858. Darwin had been working on a major book on evolution and used that to develop  On the Origins of Species , which was published in 1859. Wallace, on the other hand, continued his travels and focused his study on the importance of biogeography.

The book was not only a best seller but also one of the most influential scientific books of all time. Yet it took time for its full argument to take hold. Within a few decades, most scientists accepted that evolution and the descent of species from common ancestors were real. But natural selection had a harder time finding acceptance. In the late 1800s many scientists who called themselves Darwinists actually preferred a Lamarckian explanation for the way life changed over time. It would take the discovery of  genes  and  mutations  in the twentieth century to make natural selection not just attractive as an explanation, but unavoidable.

• More Details
• Read more about  the process of natural selection  in Evolution 101.
• Go right to the source and read Darwin's  On the Origin of Species by Means of Natural Selection .
• Explore the  American Museum of Natural History's Darwin exhibit  to learn more about his life and how his ideas transformed our understanding of the living world.

Discrete Genes Are Inherited: Gregor Mendel

Early Evolution and Development: Ernst Haeckel

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What on Earth?

Natural selection is one of the ways to account for the millions of species on Earth. For example, the beetle family Curculionidae (snout beetles) is extremely diverse, comprising an estimated 83,000 species.

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What is natural selection?

Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.

Natural selection is one of the ways to account for the millions of species that have lived on Earth.

Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) are jointly credited with coming up with the theory of evolution by natural selection, having co-published on it in 1858. Darwin has generally overshadowed Wallace since the publication of On the Origin of Species in 1859, however.

The Museum's Library holds the world's largest concentration of Darwin works, with 478 editions of On the Origin of Species in 38 languages. 

In Darwin and Wallace's time, most believed that organisms were too complex to have natural origins and must have been designed by a transcendent God. Natural selection, however, states that even the most complex organisms occur by totally natural processes.

Prof Adrian Lister , a researcher at the Museum says, 'It's not that biologists don't understand that organisms are complex and functional, and it does seem almost miraculous that they exist. We realise that, but we think we've found another way of explaining it.'

Wallace (L) and Darwin (R) came up with very similar theories on evolution. Darwin has generally overshadowed Wallace's contributions, however.

How does natural selection work?

In natural selection, genetic mutations that are beneficial to an individual's survival are passed on through reproduction. This results in a new generation of organisms that are more likely to survive to reproduce.

For example, evolving long necks has enabled giraffes to feed on leaves that others can't reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.

The evolution of a long neck is an adaptation that helps giraffes survive in their environment © FluffyCreature via Flickr ( CC BY-NC 2.0 )

Adrian explains, 'If you took 1,000 giraffes and measured their necks, they're all going to be slightly different from one another. Those differences are at least in part determined by their genes.

'The ones with longer necks may leave proportionally more offspring, because they have fed better and have maybe been better in competing for mates because they are stronger. Then, if you were to measure the necks of the next generation, they're also going to vary, but the average will have shifted slightly towards the longer ones. The process carries on generation after generation.'

What is an adaptation?

An adaptation is a physical or behavioural characteristic that helps an organism to survive in its environment. 

But not all characteristics of an animal are adaptations.

Adaptations for one purpose can be co-opted for another. For instance, feathers were an adaptation for thermoregulation - their use for flight only came later. This means that feathers are an exaptation for flight, rather than an adaptation.

Adaptations can also become outdated, such as the tough exterior of the calabash fruit ( Crescentia cujete ). This gourd is generally thought to have evolved to avoid being eaten by Gomphotheres, a family of elephant-like animals. But these animals went extinct around 10,000 years ago, so the fruit's adaptation no longer has a survival benefit. 

The large, spherical calabash fruit has an extremely tough exterior. But this adaptation is now outdated  © Wendy Cutler via Flickr ( CC BY-NC 2.0 )

Selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift or gene flow.

What does 'survival of the fittest' mean?

In terms of evolution, an animal that is 'fit' is one that is adapted to its environment. This concept is at the core of natural selection, although the term 'survival of the fittest' has often been misunderstood and may be best avoided.

There is also a degree of randomness to evolution, so the best-adapted animal won't always be the one to survive.

Adrian explains, 'If you're going to get hit by a rock or something, it's just bad luck. But on average and over time, the ones that survive are the ones that are fittest - the ones that have the best adaptations.'

Peppered moths ( Biston betularia ) are difficult to see when they perch on tree bark. Those that blend in best are less likely to be preyed on, so have advantage for survival.

What are Darwin's finches?

Darwin collected many animal specimens during the voyage of HMS Beagle (1831-1836). Among his best-known are the finches, of which he collected around 14 species from the Galápagos Islands. The birds sit within the same taxonomic family and have a diverse array of beak sizes and shapes. These correspond to both their differing primary food sources and divergence due to isolation on different islands.

The green warbler-finch ( Certhidea olivacea ), for example, has a sharp, slender beak which is perfect for feeding on small insects. In comparison, the large ground finch ( Geospiza magnirostris ) has a short, stocky beak to crack seeds and nuts. 

The Galápagos finches have distinctly different beak shapes and sizes, as can be seen here from specimens of a green warbler-finch  (L) and a large ground finch (R)

Darwin's finches are often thought of as inspiring a 'eureka moment', but it was actually mockingbirds that impacted Darwin's thoughts on evolution.

Darwin had collected mockingbirds in South America before travelling to the Galápagos. On the first island, San Cristóbal (then known as Chatham Island), he saw a bird he recognised as a mockingbird. But on nearby Floreana Island he saw that the mockingbirds were considerably different.

Darwin realised that differences between species of mockingbird on the islands were greater than between those he'd seen across the continent. He began contemplating while aboard HMS Beagle, but it took several years before he came up with his theory of evolution by natural selection.

The finches - once they had been identified as different species by the British ornithologist John Gould - became one useful example among the many other animals he saw.

Charles Darwin collected these three mockingbird specimens during his time on the Galápagos Islands in 1835, during the voyage of HMS Beagle

The finches are of scientific interest today. The study of Daphne Major , a volcanic island in the Galápagos archipelago, began in 1972 and found that natural selection has resulted in changes in the beak shape and size of two species of finch: the medium ground finch ( Geospiza fortis ) and common cactus finch ( Geospiza scandens ). Both species' beaks have been seen to shrink over time, but followed different patterns.

Darwin thought that natural selection progressed slowly and only occurred over a long period of time. This may often be true, but it has been shown that in some cases a new species can evolve within a lifetime .

For 31 years, scientists studied the survival of a male finch that emigrated from Santa Cruz Island as well as six generations of its descendants on Daphne Major. From the second generation onwards, the birds behaved as a separate species to the others on the island. 

The Daphne Major cactus finches have been studied for over 30 years. In that time the size of their beaks has fluctuated, eventually decreasing in size over a period of 15 years.

What is Lamarckism?

Lamarckism is a theory named after French naturalist Jean-Baptiste Lamarck (1744-1829). It proposes that animals acquire characteristics based on use or disuse during their lives, rather than through hard-coded genetic changes.

In Lamarckian theory, giraffes stretch their necks to make them longer. These animal's offspring would inherit longer necks as a result of their parents' efforts.

Adrian says, 'If you tried to stretch your neck for 10 minutes each morning, then you would probably end up with your neck being a few millimetres longer for a few years. But your children would not inherit it. That's where this theory fails.'

Are we still evolving?

For millennia, the world was viewed as static. The ideas that mountains could rise, and climate and organisms could change didn't exist. Earth was thought to exist in an optimal form.

But natural selection relies on the fact that the world is constantly changing. Evolution occurs automatically for survival and for millions of years it has been playing catch-up with our dynamic world. 

Poaching and habitat loss have had huge impacts on the now critically endangered saiga antelope ( Saiga tatarica ). Natural selection stands little chance in cases like this. © Andrey Giljov via Wikimedia Commons ( CC BY-SA 4.0 )

'Organisms are either adapted enough to survive and reproduce, or they are sub-optimal and the population shrinks. It may even shrink to zero, and that means extinction,' states Adrian.

Scientists have been able to predict natural selection over short terms. But it is almost impossible to accurately determine its effects in the future due to unpredictable fluctuations of the environment.

Natural selection implies that if organisms are surviving, they are adapted. But as the environment changes, we may find that what was once an adaptation may no longer be useful.

Although it is possible for evolution to occur quickly, the more rapidly the planet changes, the harder it is for evolution to keep pace and the more serious the risk of a massive rise in extinctions becomes.

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Evolution of Life

Evolution is a fundamental process that describes the changes in heritable traits of populations over successive generations. It is the mechanism by which life on Earth has diversified and adapted to various environments over millions of years. The concept of evolution is primarily associated with Charles Darwin, whose groundbreaking work, “On the Origin of Species” (1859), laid the foundation for our understanding of how species change over time through the process of natural selection.

In essence, evolution involves the transmission of genetic information from one generation to the next, with occasional mutations and variations. These variations can lead to differences in traits among individuals within a population. Over time, the traits that confer advantages in a given environment tend to be passed on more successfully, while less advantageous traits may decrease in frequency.

Significance of Studying the Evolution of Life:

• Understanding the Origin of Diversity: Studying the evolution of life provides insights into the origin and diversification of the myriad forms of life on Earth. It explains how common ancestors gave rise to the vast array of species we observe today.
• Adaptation and Natural Selection: Evolutionary theory emphasizes the role of adaptation and natural selection in shaping the characteristics of organisms. Understanding these processes is crucial for comprehending how species cope with environmental challenges and exploit available resources.
• Medical and Agricultural Applications: Knowledge of evolution is indispensable in various fields, including medicine and agriculture. It helps in understanding the emergence of diseases, the development of antibiotic resistance, and the breeding of crops with desirable traits.
• Conservation Biology: Evolutionary principles are central to conservation biology. Conservation efforts often involve preserving not only specific species but also the genetic diversity within populations to enhance their resilience in the face of environmental changes.
• Genetics and Molecular Biology: The field of genetics has greatly benefited from the insights provided by the theory of evolution. Molecular biology and genetics research often draw on evolutionary principles to understand the relationships between different species and the molecular mechanisms underlying genetic variation.
• Biogeography: The distribution of species across different regions is closely tied to their evolutionary history. Studying the evolution of life helps explain patterns of biodiversity and the factors influencing the distribution of species around the globe.
• Philosophical and Cultural Implications: Evolutionary theory has profound implications for our understanding of life’s interconnectedness and our place in the natural world. It has influenced not only scientific thought but also philosophy, ethics, and cultural perspectives on the origin and nature of life.

In summary, the study of the evolution of life is essential for comprehending the processes that have shaped the biological diversity we observe today and for addressing practical challenges in fields ranging from medicine to conservation. It serves as a unifying framework that connects various disciplines and provides a deeper understanding of the intricate web of life on Earth.

Origin of Life: Abiogenesis and the First Life Forms

Early evolutionary processes: natural selection and genetic variation, major eras in evolution, evidence of evolution: fossil record, comparative anatomy, and molecular evidence, mechanisms of evolution: genetic drift, gene flow, non-random mating, extinction events – mass extinctions, human evolution, conclusion: recap of key evolutionary milestones.

The origin of life on Earth is a complex and intriguing puzzle that scientists have been trying to unravel for many years. The leading scientific hypothesis for the origin of life is abiogenesis, which proposes that life arose from non-living matter under the right conditions.

Abiogenesis: Abiogenesis, also known as spontaneous generation, is the process by which living organisms are thought to have arisen from non-living matter. The transition from simple organic molecules to self-replicating, life-sustaining entities is a critical aspect of abiogenesis. While the details of how abiogenesis occurred remain uncertain, several key steps are often considered:

• Formation of Simple Organic Molecules: The early Earth had a reducing atmosphere, and various experiments have demonstrated that simple organic molecules, such as amino acids and nucleotides, could have formed under conditions resembling those of the early Earth. These molecules are the building blocks of life.
• Formation of Polymers: Simple organic molecules could have polymerized to form more complex structures, such as proteins and nucleic acids. This process may have occurred in the oceans or other prebiotic environments.
• Formation of Protocells: Protocells are hypothesized to be precursors to modern cells. These structures would have had a lipid membrane or some other boundary that separated their internal environment from the external surroundings. They might have exhibited basic cellular properties, such as the ability to maintain internal chemistry distinct from the external environment.
• Development of Self-Replication: One of the defining features of life is the ability to replicate. The transition from non-living to living entities likely involved the development of mechanisms for self-replication, allowing the information stored in molecules like RNA to be passed on to subsequent generations.

While the details of these steps are still the subject of ongoing research and debate, the general framework of abiogenesis provides a plausible explanation for how life could have originated from non-living matter on Earth.

First Life Forms: Determining what the first life forms were is challenging because they would have been simple and lacked many of the complex features of modern organisms. The transition from simple organic compounds to the first living entities was likely gradual.

RNA World Hypothesis: The RNA world hypothesis suggests that early life forms were based on RNA (ribonucleic acid) rather than DNA. RNA is capable of both storing genetic information and catalyzing chemical reactions, making it a plausible candidate for the first self-replicating molecules.

The journey from the prebiotic conditions of early Earth to the emergence of the first life forms remains one of the most captivating questions in scientific inquiry. Ongoing research in fields such as biochemistry, molecular biology, and astrobiology continues to shed light on the fascinating process by which life may have originated on our planet.

The early evolutionary processes that shaped life on Earth were driven by mechanisms such as natural selection and genetic variation. These processes laid the foundation for the diversity of life we observe today.

Natural Selection: Natural selection is a fundamental mechanism of evolution proposed by Charles Darwin. It describes the process by which organisms with traits that better suit their environment tend to survive and reproduce more successfully than those with less advantageous traits. Over time, the frequency of advantageous traits in a population increases, leading to the adaptation of species to their environments.

Key principles of natural selection include:

• Variation: Within any population, there is genetic variation, meaning that individuals in a species can exhibit different traits. This variation can arise through mutations, genetic recombination, and other mechanisms.
• Heritability: The traits that provide a reproductive advantage are often heritable, meaning they can be passed down from one generation to the next through genetic information.
• Differential Reproduction: Organisms with advantageous traits are more likely to survive and reproduce, passing those traits on to their offspring. Over time, this leads to an increase in the frequency of these traits in the population.
• Adaptation: As a result of natural selection, populations become better adapted to their environments. This adaptation can occur at various levels, from specific traits that improve survival to more complex adaptations that enhance reproductive success.

Genetic Variation: Genetic variation is the raw material upon which natural selection acts. It is the diversity in the genetic makeup of individuals within a population. This variation arises through processes such as:

• Mutation: Mutations are random changes in an organism’s DNA sequence. They can occur due to various factors, such as errors during DNA replication, exposure to radiation, or certain chemicals. Mutations introduce new genetic material, contributing to the diversity of traits within a population.
• Recombination: During sexual reproduction, genetic material from two parent organisms is combined to produce offspring with a unique combination of genes. This process, known as genetic recombination, further increases genetic diversity.
• Gene Flow: Gene flow occurs when individuals or their gametes move between populations, introducing new genetic material. This can happen through migration or other mechanisms that allow genetic exchange between different groups of organisms.

Early Evolutionary Events: In the early stages of evolution, simple organisms underwent processes of natural selection and genetic variation. The emergence of self-replicating molecules, the development of cellular structures, and the evolution of metabolic processes were crucial milestones. Over time, the complexity of life increased as organisms adapted to different ecological niches.

These early evolutionary processes set the stage for the incredible diversity of life that has evolved on Earth. The interplay between natural selection and genetic variation continues to shape the characteristics of living organisms, influencing their ability to survive and reproduce in changing environments.

The history of life on Earth is often divided into several major eras based on significant evolutionary events and changes in the composition of the Earth’s biota. These divisions help scientists organize the vast timeline of life into more manageable units. The major eras in evolution are typically grouped into the following:

• Hadean Eon (4.6 to 4.0 billion years ago): This era represents the earliest period of Earth’s history, characterized by the formation of the planet from the solar nebula. Conditions during the Hadean Eon were extremely harsh, with high temperatures and frequent impacts from celestial bodies.
• Archean Eon (4.0 to 2.5 billion years ago): During the Archean Eon, the Earth’s surface began to cool, and the first continents and oceans formed. Simple life forms, such as bacteria and archaea, likely originated during this time.
• Proterozoic Eon (2.5 billion years ago to 541 million years ago): The Proterozoic Eon saw the evolution of more complex single-celled organisms, including eukaryotes. Towards the end of this eon, there was a significant increase in the complexity of multicellular life.
• The Paleozoic Era is often referred to as the “Age of Invertebrates” and the “Age of Fishes.” It witnessed the development of various marine invertebrates, fish, and the first land-dwelling plants and animals.
• Significant events include the Cambrian Explosion, during which a diverse array of animal phyla appeared, and the colonization of land by plants and arthropods.
• The Paleozoic Era concludes with the Permian-Triassic Extinction Event, one of the most significant mass extinctions in Earth’s history.
• The Mesozoic Era is often referred to as the “Age of Reptiles” and is divided into three periods: the Triassic, Jurassic, and Cretaceous.
• Dinosaurs, including iconic species like Tyrannosaurus rex and Velociraptor, dominated terrestrial ecosystems. Marine reptiles, such as ichthyosaurs and plesiosaurs, flourished in the oceans.
• The Mesozoic Era concludes with the Cretaceous-Paleogene Extinction Event, which led to the extinction of the dinosaurs and the rise of mammals.
• The Cenozoic Era is often called the “Age of Mammals” and is the current era. It is divided into the Paleogene, Neogene, and Quaternary periods.
• Mammals diversified and became the dominant terrestrial vertebrates. The evolution of primates eventually led to the emergence of humans.
• The Quaternary period includes the Pleistocene epoch, marked by repeated glaciations, and the Holocene epoch, which encompasses the last approximately 11,700 years, representing the period of human civilization.

These major eras provide a framework for understanding the long and dynamic history of life on Earth, from the earliest single-celled organisms to the complex and diverse ecosystems observed today.

The theory of evolution is supported by a diverse range of evidence that spans multiple scientific disciplines. Three key types of evidence include the fossil record, comparative anatomy, and molecular evidence.

• Fossils are preserved remains or traces of organisms from the past. The fossil record provides a historical snapshot of life on Earth and is a crucial source of evidence for evolution.
• Transitional Fossils: Transitional fossils are intermediate forms that show characteristics of both ancestral and derived groups. Examples include Tiktaalik, a fish-like tetrapod precursor.
• Stratigraphy : The arrangement of fossils in rock layers (strata) provides a chronological record. Deeper layers generally contain older fossils, allowing scientists to observe changes over time.
• Comparative anatomy involves the study of the similarities and differences in the structures of organisms. These comparisons reveal evolutionary relationships and adaptations.
• Homologous Structures: Structures that have a common evolutionary origin, even if they serve different functions in different organisms. For example, the pentadactyl limb structure in vertebrates.
• Analogous Structures: Structures that have similar functions but different evolutionary origins. This is often a result of convergent evolution, where unrelated organisms evolve similar traits due to similar environmental pressures.
• Molecular biology has provided powerful evidence for evolution by examining the genetic material of organisms.
• DNA Sequencing: By comparing DNA sequences, scientists can determine the degree of genetic similarity between different species. The more closely related two species are, the more similar their DNA sequences.
• Genetic Homologies: Similarities in the DNA sequences of genes across different species provide evidence of shared ancestry. Conserved genes are often crucial for basic cellular functions.
• Pseudogenes and Retroviruses: The presence of shared pseudogenes (non-functional DNA sequences) and retroviral DNA in the genomes of different species can indicate a common evolutionary origin.
• The distribution of species around the globe supports the idea of evolution. Similar environments often host species with similar adaptations, even if they are not closely related phylogenetically.
• Endemism: The presence of species unique to specific geographic regions is consistent with the idea that species evolve in response to local conditions.
• The study of embryonic development provides insights into evolutionary relationships. Similarities in the early developmental stages of different organisms suggest common ancestry.
• Artificial Selection: Selective breeding by humans, as seen in domesticated plants and animals, mimics the process of natural selection. It demonstrates how specific traits can be accentuated over generations.
• Observations of Evolution in Action: Examples of observable evolution, such as antibiotic resistance in bacteria or changes in the beak size of Darwin’s finches in response to environmental conditions, provide real-time evidence for evolutionary processes.

By examining these various lines of evidence, scientists can build a comprehensive understanding of the processes and patterns of evolution, supporting the overarching theory proposed by Charles Darwin and Alfred Russel Wallace in the 19th century.

Evolution is driven by various mechanisms that act on populations and their genetic composition over time. Three important mechanisms are genetic drift, gene flow, and non-random mating.

• Genetic drift refers to the random fluctuations in the frequency of alleles in a population over generations. It is particularly influential in small populations.
• Bottleneck Effect: Occurs when a population is sharply reduced in size, leading to a significant loss of genetic diversity. The surviving population may have a gene pool that differs from the original population.
• Founder Effect: Occurs when a small group of individuals establishes a new population, and the gene pool of this founding group may not represent the genetic diversity of the larger source population.
• Gene flow, also known as migration or gene migration, is the movement of genes between populations. It occurs when individuals migrate and interbreed with members of other populations.
• Homogenizing Effect: Gene flow tends to reduce genetic differences between populations over time. It can introduce new alleles to a population or reduce the frequency of existing alleles.
• Isolating Mechanisms: In contrast to homogenization, gene flow can be restricted by geographic, ecological, or reproductive barriers, contributing to the divergence of populations.
• Non-random mating occurs when individuals choose mates based on specific traits or when mating is not purely a chance process. This can lead to changes in the frequency of alleles in a population.
• Assortative Mating: Individuals with similar traits are more likely to mate with each other. This can increase the frequency of certain alleles in a population.
• Dissassortative Mating: Individuals with dissimilar traits are more likely to mate. This can lead to the maintenance of genetic diversity in a population.

These mechanisms, along with natural selection and mutation, contribute to the genetic diversity and adaptation of populations over time. It’s important to note that these processes can interact, and their effects may vary depending on the specific characteristics of a population and its environment.

In summary, genetic drift, gene flow, and non-random mating are important factors influencing the genetic makeup of populations and play significant roles in the evolutionary process. Together, these mechanisms contribute to the ongoing changes and diversity observed in living organisms.

Extinction events are periods in Earth’s history during which a significant number of species go extinct in a relatively short geological time span. Mass extinctions are particularly dramatic events that result in the loss of a substantial proportion of Earth’s biodiversity. Throughout the history of life on Earth, there have been several mass extinctions, each marking the end of an era and the beginning of new evolutionary trajectories. The five most well-known mass extinctions are often referred to as the “Big Five.”

• This early mass extinction event primarily affected marine life, particularly brachiopods and bryozoans.
• The causes are not fully understood, but potential factors include changes in sea levels and glaciation.
• This extinction event had a significant impact on marine life, especially reef-building organisms like corals and stromatoporoids.
• Possible causes include climate change, sea-level fluctuations, and the evolution of land plants affecting marine ecosystems.
• Often referred to as the “Great Dying,” this is the most severe mass extinction in Earth’s history, resulting in the loss of approximately 96% of marine species and 70% of terrestrial vertebrate species.
• Causes are debated but may include volcanic activity, climate change, and ocean anoxia (lack of oxygen).
• This extinction event affected marine and terrestrial life, including some large amphibians and reptiles.
• Possible causes include volcanic activity, climate change, and the opening of the Atlantic Ocean.
• This is the most well-known mass extinction event and marks the end of the Mesozoic Era. It resulted in the extinction of approximately 75% of Earth’s species, including the non-avian dinosaurs.
• The impact hypothesis suggests that a large asteroid or comet impact contributed to the extinction, along with volcanic activity and other environmental changes.

Significance of Mass Extinctions:

• Mass extinctions have profound effects on the course of evolution, as they create ecological vacancies that can be filled by new species.
• They mark the end of one era and the beginning of another, with surviving species evolving to occupy available niches.
• Mass extinctions are key events in Earth’s geological and biological history, shaping the diversity and composition of life on the planet.

While mass extinctions are associated with catastrophic events, it’s important to note that ongoing extinctions, often driven by human activities, are occurring at an accelerated rate and are a significant concern for biodiversity and ecosystem health.

Human evolution is the evolutionary process that led to the emergence of Homo sapiens, the anatomically modern human species. The timeline of human evolution spans millions of years and involves various species and hominids (members of the biological family Hominidae).

Australopithecines (4 to 2 million years ago):

The Australopithecines were bipedal primates that lived in Africa. The most famous Australopithecine is Lucy (Australopithecus afarensis). Bipedalism (walking on two legs) is a key trait that distinguishes hominids from other primates.

Genus Homo (2.4 to 2 million years ago):

Homo habilis is one of the earliest members of the Homo genus, known for its use of stone tools. This period marks the beginning of the Oldowan tool culture.

Homo erectus (1.9 million to 140,000 years ago):

Homo erectus is characterized by a larger brain size, more advanced tools (Acheulean tools), and the ability to control fire. They were also the first hominids to migrate out of Africa, spreading into Asia and Europe.

Archaic Homo sapiens (500,000 to 200,000 years ago):

This category includes various hominid species that share characteristics with both Homo erectus and anatomically modern Homo sapiens. Notable examples include Homo heidelbergensis.

Homo sapiens (around 300,000 years ago to present):

Anatomically modern Homo sapiens emerged in Africa and gradually spread across the globe. Behavioral and cultural innovations, including complex tool use, art, and symbolic thinking, distinguish Homo sapiens from earlier hominids.

Cultural Evolution:

Cultural evolution refers to the adaptive changes in the shared knowledge, beliefs, and behaviors of human societies over time. Unlike biological evolution, which operates on genetic information, cultural evolution involves the transmission of information through social learning, language, and symbolic communication.

• The development of language enabled humans to transmit complex ideas, facilitating the accumulation and transmission of cultural knowledge.
• The ability to create and use tools is a defining feature of human cultural evolution. Technological advancements have played a crucial role in human survival and adaptation.
• Human societies evolved from small groups to complex social structures. The development of agriculture and settled communities marked a significant shift in social organization.
• The creation of art and symbolic representations reflects the cognitive complexity of human cultures. Cave paintings, sculptures, and other forms of artistic expression provide insights into the beliefs and values of ancient societies.
• Human cultures have diversified in response to environmental conditions, geographic isolation, and historical factors. Cultural diversity is a testament to the adaptability and creativity of human societies.

Understanding human evolution and cultural evolution provides valuable insights into the development of our species and the factors that have shaped our biological and cultural diversity. It also highlights the dynamic interplay between biological and cultural factors in the evolution of Homo sapiens.

The story of evolution is a captivating journey that spans billions of years, marked by key milestones and events that have shaped the incredible diversity of life on Earth. Here is a recap of some key evolutionary milestones:

• Abiogenesis, the emergence of life from non-living matter, set the stage for the evolutionary process.
• Natural selection and genetic variation drove the development of simple life forms, leading to the emergence of increasingly complex organisms.
• The Precambrian, Paleozoic, Mesozoic, and Cenozoic eras witnessed significant evolutionary changes, from the emergence of multicellular life to the dominance of dinosaurs and the rise of mammals.
• The fossil record, comparative anatomy, molecular evidence, biogeography, embryology, and observational evidence collectively provide robust support for the theory of evolution.
• Genetic drift, gene flow, non-random mating, natural selection, and mutation are fundamental mechanisms that drive evolutionary change in populations.
• Five major mass extinctions, including the Permian-Triassic and Cretaceous-Paleogene extinctions, significantly influenced the course of evolution by shaping biodiversity and opening ecological niches.
• The evolutionary journey of hominids, from Australopithecines to modern Homo sapiens, is characterized by the development of bipedalism, tool use, increased brain size, and the emergence of complex societies.
• The evolution of human cultures involves language development, tool use, social organization, art, and symbolic thinking. Cultural evolution complements biological evolution and plays a crucial role in human adaptability.

Ongoing Research and Future Directions:

• Continued advancements in genomics and molecular biology allow scientists to explore the genetic basis of evolution in unprecedented detail. Comparative genomics and the study of functional genomics contribute to our understanding of genetic variation and adaptation.
• The field of paleogenomics involves extracting and analyzing ancient DNA from fossils. This allows researchers to gain insights into the genomes of extinct species and understand genetic changes over time.
• Ongoing research focuses on understanding how ecological and climate changes influence evolutionary processes. This includes studying the impact of human activities on biodiversity and ecosystems.
• Interdisciplinary approaches that integrate data from paleontology , genetics, ecology, and other fields provide a more comprehensive understanding of evolutionary processes and their outcomes.
• Studying contemporary examples of evolution in action, such as antibiotic resistance in bacteria, provides insights into the dynamics of natural selection and adaptation in real-time.
• Researchers continue to explore the origins of life, with a focus on understanding the conditions that led to the emergence of the first living organisms on Earth.
• Evolutionary research raises ethical questions and societal implications. Ongoing discussions involve the integration of scientific knowledge into education, public policy, and ethical considerations related to genetic technologies.

The study of evolution remains a dynamic and evolving field, continually expanding our understanding of the processes that have shaped life on Earth. As technology advances and new discoveries are made, the future of evolutionary research holds the promise of further unraveling the mysteries of life’s intricate tapestry.

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2.1: The Theory of Natural Selection

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The theory of evolution by natural selection describes a mechanism for species change over time. That species change had been suggested and debated well before Darwin. The view that species were unchanging was grounded in the writings of Plato, yet there were other ancient Greeks philosophers that expressed evolutionary ideas.

Definition: Natural Selection

The process by which living organisms adapt and change in nature. Natural selection results from differential survival and reproduction of individuals due to differences in heritable traits. It is a key mechanism of evolution and is the only process that results in the formation of adaptive traits and behaviors. 

In the eighteenth century, ideas about the evolution of animals were reintroduced by the naturalist Georges-Louis Leclerc, Comte de Buffon and even by Charles Darwin’s grandfather, Erasmus Darwin. During this time, it was also accepted that there were extinct species. At the same time, James Hutton, the Scottish naturalist, proposed that geological change occurred gradually by the accumulation of small changes from processes (over long periods of time) just like those happening today. This contrasted with the predominant view that the geology of the planet was a consequence of catastrophic events occurring during a relatively brief past. Hutton’s view was later popularized by the geologist Charles Lyell in the nineteenth century. Lyell became a friend to Darwin and his ideas were very influential on Darwin’s thinking. Lyell argued that the greater age of Earth gave more time for gradual change in species, and the process provided an analogy for gradual change in species.

In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change that is now referred to as inheritance of acquired characteristics. In Lamarck’s theory, modifications in an individual caused by its environment, or the use or disuse of a structure during its lifetime, could be inherited by its offspring and, thus, bring about change in a species. While this mechanism for evolutionary change as described by Lamarck was discredited, Lamarck’s ideas were an important influence on evolutionary thought. 

Charles Darwin and Alfred Wallace

The actual mechanism for evolution was independently conceived of and described by two naturalists, Charles Darwin and Alfred Russell Wallace, in the mid-nineteenth century. Importantly, each spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle , visiting South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys in the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands (west of Ecuador). On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species that each had a unique beak shape. He observed both that these finches closely resembled another finch species on the mainland of South America and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with very small differences between the most similar. Darwin imagined that the island species might be all species modified from one original mainland species. In 1860, he wrote, “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends." (Darwin, 1860). 

The Theory of Natural Selection

Wallace and Darwin both observed similar patterns in other organisms and independently conceived a mechanism to explain how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature:

• First, the characteristics of organisms are inherited, or passed from parent to offspring.
• Second, more offspring are produced than are able to survive; in other words, resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers.
• Thus, there is a competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading an essay by the economist Thomas Malthus, who discussed this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited.

Out of these three principles, Darwin and Wallace reasoned that offspring with inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called “descent with modification.”

Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnaean Society in London. The following year Darwin’s book, On the Origin of Species, was published, which outlined in considerable detail his arguments for evolution by natural selection.

Variation and Adaptation

Natural selection can only take place if there is  variation , or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons, such as an individual being taller because of better nutrition rather than different genes. The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation (changes in DNA sequence). 

A heritable trait (one that is genetically, rather than environmentally determined) that aids the survival and reproduction of an organism in its present environment is called an  adaptation . An adaptation is a “match” of the organism to the environment. Adaptation to an environment comes about when a change in the range of genetic variation occurs over time that increases or maintains the match of the population with its environment. 

Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Whether or not a trait is favorable depends on the environment at the time. Traits that are helpful in one environment might actually be harmful in another. 

Modern Evidence of Natural Selection in Galapagos Finches

Demonstrations of evolution by natural selection can take years to study. One of the best lines of evidence has been in the very birds that helped to inspire the theory, the Galápagos finches. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976. The Grants found changes from one generation to the next in the beak shapes of the medium ground finches on the Galápagos island of Daphne Major. The medium ground finch feeds on seeds. The birds have inherited variation in the genes for bill shape. Some individuals have an allele (a variation of the gene for bill shape) that causes a wide, deep bill, and others have an allele for thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During 1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically: the decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year. When the Grants measured beak sizes in the much-reduced population the year after the drought, they found that the average bill size was larger. This was clear evidence for natural selection (differences in survival) of bill size caused by the availability of seeds. The Grants had studied the inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and evolution of bill size in this species in response to changing conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare.

Darwin, Charles. 1860. Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the World, under the Command of Capt. Fitz Roy, R.N , 2nd. ed. (London: John Murray, 1860), http://www.archive.org/details/journalofresea00darw .

Home — Essay Samples — Science — Biology — Natural Selection

Essays on Natural Selection

Prompt examples for natural selection essays, the theory of natural selection.

Explain Charles Darwin's theory of natural selection and how it serves as a cornerstone of modern evolutionary biology.

Examples of Natural Selection in the Wild

Provide real-world examples of natural selection in action, highlighting specific species and adaptations that have evolved due to natural selection.

Comparative Anatomy and Homology

Discuss the concept of comparative anatomy and homology as evidence for evolution through natural selection, focusing on shared anatomical features among different species.

Adaptive Radiation and Speciation

Explore the process of adaptive radiation and how it leads to speciation, using examples from the natural world to illustrate this phenomenon.

The Role of Genetic Variation

Analyze the importance of genetic variation in the context of natural selection, including how mutations and genetic diversity contribute to evolutionary change.

Sexual Selection and Mate Choice

Examine the concept of sexual selection and how it influences the evolution of traits related to mating and reproduction in various species.

Human Evolution and Natural Selection

Discuss the application of natural selection in human evolution, including adaptations and traits that have shaped the human species.

Ecological Factors and Natural Selection

Analyze how ecological factors, such as competition for resources and environmental changes, drive natural selection and influence the evolution of species.

Evidence from Fossil Records

Examine the evidence for evolution and natural selection found in the fossil record, including transitional fossils and the documentation of evolutionary history.

The Role of Geographic Isolation

Discuss how geographic isolation and allopatric speciation contribute to the diversification of species and the formation of new ones.

Natural Selection Lab Report

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Volume 2 Supplement 2

Special Issue: Transitional Fossils

• Evolutionary Concepts
• Open access
• Published: 09 April 2009

Understanding Natural Selection: Essential Concepts and Common Misconceptions

• T. Ryan Gregory 1  

Evolution: Education and Outreach volume  2 ,  pages 156–175 ( 2009 ) Cite this article

135 Citations

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Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Without a working knowledge of natural selection, it is impossible to understand how or why living things have come to exhibit their diversity and complexity. An understanding of natural selection also is becoming increasingly relevant in practical contexts, including medicine, agriculture, and resource management. Unfortunately, studies indicate that natural selection is generally very poorly understood, even among many individuals with postsecondary biological education. This paper provides an overview of the basic process of natural selection, discusses the extent and possible causes of misunderstandings of the process, and presents a review of the most common misconceptions that must be corrected before a functional understanding of natural selection and adaptive evolution can be achieved.

“There is probably no more original, more complex, and bolder concept in the history of ideas than Darwin's mechanistic explanation of adaptation.” Ernst Mayr ( 1982 , p.481)

Introduction

Natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next. That this process can be encapsulated within a single (admittedly lengthy) sentence should not diminish the appreciation of its profundity and power. It is one of the core mechanisms of evolutionary change and is the main process responsible for the complexity and adaptive intricacy of the living world. According to philosopher Daniel Dennett ( 1995 ), this qualifies evolution by natural selection as “the single best idea anyone has ever had.”

Natural selection results from the confluence of a small number of basic conditions of ecology and heredity. Often, the circumstances in which those conditions apply are of direct significance to human health and well-being, as in the evolution of antibiotic and pesticide resistance or in the impacts of intense predation by humans (e.g., Palumbi 2001 ; Jørgensen et al. 2007 ; Darimont et al. 2009 ). Understanding this process is therefore of considerable importance in both academic and pragmatic terms. Unfortunately, a growing list of studies indicates that natural selection is, in general, very poorly understood—not only by young students and members of the public but even among those who have had postsecondary instruction in biology.

As is true with many other issues, a lack of understanding of natural selection does not necessarily correlate with a lack of confidence about one's level of comprehension. This could be due in part to the perception, unfortunately reinforced by many biologists, that natural selection is so logically compelling that its implications become self-evident once the basic principles have been conveyed. Thus, many professional biologists may agree that “[evolution] shows how everything from frogs to fleas got here via a few easily grasped biological processes ” (Coyne 2006 ; emphasis added). The unfortunate reality, as noted nearly 20 years ago by Bishop and Anderson ( 1990 ), is that “the concepts of evolution by natural selection are far more difficult for students to grasp than most biologists imagine.” Despite common assumptions to the contrary by both students and instructors, it is evident that misconceptions about natural selection are the rule, whereas a working understanding is the rare exception.

The goal of this paper is to enhance (or, as the case may be, confirm) readers' basic understanding of natural selection. This first involves providing an overview of the basis and (one of the) general outcomes of natural selection as they are understood by evolutionary biologists Footnote 1 . This is followed by a brief discussion of the extent and possible causes of difficulties in fully grasping the concept and consequences of natural selection. Finally, a review of the most widespread misconceptions about natural selection is provided. It must be noted that specific instructional tools capable of creating deeper understanding among students generally have remained elusive, and no new suggestions along these lines are presented here. Rather, this article is aimed at readers who wish to confront and correct any misconceptions that they may harbor and/or to better recognize those held by most students and other non-specialists.

The Basis and Basics of Natural Selection

Though rudimentary forms of the idea had been presented earlier (e.g., Darwin and Wallace 1858 and several others before them), it was in On the Origin of Species by Means of Natural Selection that Darwin ( 1859 ) provided the first detailed exposition of the process and implications of natural selection Footnote 2 . According to Mayr ( 1982 , 2001 ), Darwin's extensive discussion of natural selection can be distilled to five “facts” (i.e., direct observations) and three associated inferences. These are depicted in Fig.  1 .

The basis of natural selection as presented by Darwin ( 1859 ), based on the summary by Mayr ( 1982 )

Some components of the process, most notably the sources of variation and the mechanisms of inheritance, were, due to the limited available information in Darwin's time, either vague or incorrect in his original formulation. Since then, each of the core aspects of the mechanism has been elucidated and well documented, making the modern theory Footnote 3 of natural selection far more detailed and vigorously supported than when first proposed 150 years ago. This updated understanding of natural selection consists of the elements outlined in the following sections.

Overproduction, Limited Population Growth, and the “Struggle for Existence”

A key observation underlying natural selection is that, in principle, populations have the capacity to increase in numbers exponentially (or “geometrically”). This is a simple function of mathematics: If one organism produces two offspring, and each of them produces two offspring, and so on, then the total number grows at an increasingly rapid rate (1 → 2 → 4 → 8 → 16 → 32 → 64... to 2 n after n rounds of reproduction).

The enormity of this potential for exponential growth is difficult to fathom. For example, consider that beginning with a single Escherichia coli bacterium, and assuming that cell division occurs every 30 minutes, it would take less than a week for the descendants of this one cell to exceed the mass of the Earth. Of course, exponential population expansion is not limited to bacteria. As Nobel laureate Jacques Monod once quipped, “What is true for E. coli is also true for the elephant,” and indeed, Darwin ( 1859 ) himself used elephants as an illustration of the principle of rapid population growth, calculating that the number of descendants of a single pair would swell to more than 19,000,000 in only 750 years Footnote 4 . Keown ( 1988 ) cites the example of oysters, which may produce as many as 114,000,000 eggs in a single spawn. If all these eggs grew into oysters and produced this many eggs of their own that, in turn, survived to reproduce, then within five generations there would be more oysters than the number of electrons in the known universe.

Clearly, the world is not overrun with bacteria, elephants, or oysters. Though these and all other species engage in massive overproduction (or “superfecundity”) and therefore could in principle expand exponentially, in practice they do not Footnote 5 . The reason is simple: Most offspring that are produced do not survive to produce offspring of their own. In fact, most population sizes tend to remain relatively stable over the long term. This necessarily means that, on average, each pair of oysters produces only two offspring that go on to reproduce successfully—and that 113,999,998 eggs per female per spawn do not survive (see also Ridley 2004 ). Many young oysters will be eaten by predators, others will starve, and still others will succumb to infection. As Darwin ( 1859 ) realized, this massive discrepancy between the number of offspring produced and the number that can be sustained by available resources creates a “struggle for existence” in which often only a tiny fraction of individuals will succeed. As he noted, this can be conceived as a struggle not only against other organisms (especially members of the same species, whose ecological requirements are very similar) but also in a more abstract sense between organisms and their physical environments.

Variation and Inheritance

Variation among individuals is a fundamental requirement for evolutionary change. Given that it was both critical to his theory of natural selection and directly counter to much contemporary thinking, it should not be surprising that Darwin ( 1859 ) expended considerable effort in attempting to establish that variation is, in fact, ubiquitous. He also emphasized the fact that some organisms—namely relatives, especially parents and their offspring—are more similar to each other than to unrelated members of the population. This, too, he realized is critical for natural selection to operate. As Darwin ( 1859 ) put it, “Any variation which is not inherited is unimportant for us.” However, he could not explain either why variation existed or how specific characteristics were passed from parent to offspring, and therefore was forced to treat both the source of variation and the mechanism of inheritance as a “black box.”

The workings of genetics are no longer opaque. Today, it is well understood that inheritance operates through the replication of DNA sequences and that errors in this process (mutations) and the reshuffling of existing variants (recombination) represent the sources of new variation. In particular, mutations are known to be random (or less confusingly, “undirected”) with respect to any effects that they may have. Any given mutation is merely a chance error in the genetic system, and as such, its likelihood of occurrence is not influenced by whether it will turn out to be detrimental, beneficial, or (most commonly) neutral.

As Darwin anticipated, extensive variation among individuals has now been well established to exist at the physical, physiological, and behavioral levels. Thanks to the rise of molecular biology and, more recently, of genomics, it also has been possible to document variation at the level of proteins, genes, and even individual DNA nucleotides in humans and many other species.

Non-random Differences in Survival and Reproduction

Darwin saw that overproduction and limited resources create a struggle for existence in which some organisms will succeed and most will not. He also recognized that organisms in populations differ from one another in terms of many traits that tend to be passed on from parent to offspring. Darwin's brilliant insight was to combine these two factors and to realize that success in the struggle for existence would not be determined by chance, but instead would be biased by some of the heritable differences that exist among organisms. Specifically, he noted that some individuals happen to possess traits that make them slightly better suited to a particular environment, meaning that they are more likely to survive than individuals with less well suited traits. As a result, organisms with these traits will, on average, leave more offspring than their competitors.

Whereas the origin of a new genetic variant occurs at random in terms of its effects on the organism, the probability of it being passed on to the next generation is absolutely non-random if it impacts the survival and reproductive capabilities of that organism. The important point is that this is a two-step process: first, the origin of variation by random mutation, and second, the non-random sorting of variation due to its effects on survival and reproduction (Mayr 2001 ). Though definitions of natural selection have been phrased in many ways (Table  1 ), it is this non-random difference in survival and reproduction that forms the basis of the process.

Darwinian Fitness

The meaning of fitness in evolutionary biology.

In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype (gene complement), phenotype (physical and behavioral features), survival, and reproduction in particular environments. The concept used by evolutionary biologists in this regard is known as “Darwinian fitness,” which is defined most simply as a measure of the total (or relative) reproductive output of an organism with a particular genotype (Table  1 ). In the most basic terms, one can state that the more offspring an individual produces, the higher is its fitness. It must be emphasized that the term “fitness,” as used in evolutionary biology, does not refer to physical condition, strength, or stamina and therefore differs markedly from its usage in common language.

“Survival of the Fittest” is Misleading

In the fifth edition of the Origin (published in 1869), Darwin began using the phrase “survival of the fittest”, which had been coined a few years earlier by British economist Herbert Spencer, as shorthand for natural selection. This was an unfortunate decision as there are several reasons why “survival of the fittest” is a poor descriptor of natural selection. First, in Darwin's context, “fittest” implied “best suited to a particular environment” rather than “most physically fit,” but this crucial distinction is often overlooked in non-technical usage (especially when further distorted to “only the strong survive”). Second, it places undue emphasis on survival: While it is true that dead organisms do not reproduce, survival is only important evolutionarily insofar as it affects the number of offspring produced. Traits that make life longer or less difficult are evolutionarily irrelevant unless they also influence reproductive output. Indeed, traits that enhance net reproduction may increase in frequency over many generations even if they compromise individual longevity. Conversely, differences in fecundity alone can create differences in fitness, even if survival rates are identical among individuals. Third, this implies an excessive focus on organisms, when in fact traits or their underlying genes equally can be identified as more or less fit than alternatives. Lastly, this phrase is often misconstrued as being circular or tautological (Who survives? The fittest. Who are the fittest? Those who survive). However, again, this misinterprets the modern meaning of fitness, which can be both predicted in terms of which traits are expected to be successful in a specific environment and measured in terms of actual reproductive success in that environment.

Which Traits Are the Most Fit?

Directional natural selection can be understood as a process by which fitter traits (or genes) increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment. Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now —natural selection cannot increase the proportion of traits solely because they may someday become advantageous. Careful reflection on how natural selection actually works should make it clear why this is so.

Natural Selection and Adaptive Evolution

Natural selection and the evolution of populations.

Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6 . Overproduction alone will have no evolutionary consequences if all individuals are identical. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction. However, any time all of Darwin's postulates hold simultaneously—as they do in most populations—natural selection will occur. The net result in this case is that certain traits (or, more precisely, genetic variants that specify those traits) will, on average , be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.

The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits. If this process happens to occur in a consistent direction—say, the largest individuals in each generation tend to leave more offspring than smaller individuals—then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population. Organisms do not evolve; populations evolve.

The term “adaptation” derives from ad + aptus , literally meaning “toward + fit”. As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. On a broader scale, it is also how physical, physiological, and behavioral features that contribute to survival and reproduction (“adaptations”) arise over evolutionary time. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence. (For a detailed discussion of the evolution of complex organs such as eyes, see Gregory 2008b .)

On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact (as many readers will have surmised), most forms of natural selection deplete genetic variation within populations. How, then, can an eliminative process like natural selection ever lead to creative outcomes?

To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation. Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection (but not, it must be pointed out, to evolution more broadly). The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations. However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them. Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations.

Biologists sometimes describe beneficial mutations as “spreading” or “sweeping” through a population, but this shorthand is misleading. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them. Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. At this point, that beneficial genetic variant is said to have become “fixed” in the population.

Again, mutation does not occur in order to improve fitness—it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed. As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.

Of course, this is an oversimplification—in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially. Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually (though perhaps very infrequently and with only minor positive impacts) and then accumulate in the population over many generations.

The process of adaptation in a population is depicted in very basic form in Fig.  2 . Several important points can be drawn from even such an oversimplified rendition:

Mutations are the source of new variation. Natural selection itself does not create new traits; it only changes the proportion of variation that is already present in the population. The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features.

Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening “by chance”.

Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.

No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations.

The direction in which adaptive change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa.

Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features (see Gregory 2008b ).

It does not matter what an “ideal” adaptive feature might be—the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.”

The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past.

A highly simplified depiction of natural selection ( Correct ) and a generalized illustration of various common misconceptions about the mechanism ( Incorrect ). Properly understood, natural selection occurs as follows: ( A ) A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true. As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits ( B ). Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. ( C ) Because the trait in question is heritable, this second generation will (mostly) resemble the parent generation. However, mutations have also occurred, which are undirected (i.e., they occur at random in terms of the consequences of changing traits), leading to both lighter and darker offspring in Generation 2 as compared to their parents in Generation 1. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals ( D ). This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring ( E ). ( F ) This process of undirected mutation and natural selection (non-random differences in survival and reproductive success) occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation. By Generation N , the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists. In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm ( X ). It is assumed that all members within a single generation change in response to pressures imposed by the environment ( Y ). When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement ( Z ). Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism. Diagrams based in part on Bishop and Anderson ( 1990 )

Natural Selection Is Elegant, Logical, and Notoriously Difficult to Grasp

The extent of the problem.

In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. As a mechanism, it is remarkably simple in principle yet incredibly powerful in application. However, the fact that it eluded description until 150 years ago suggests that grasping its workings and implications is far more challenging than is usually assumed.

Three decades of research have produced unambiguous data revealing a strikingly high prevalence of misconceptions about natural selection among members of the public and in students at all levels, from elementary school pupils to university science majors (Alters 2005 ; Bardapurkar 2008 ; Table  2 ) Footnote 7 . A finding that less than 10% of those surveyed possess a functional understanding of natural selection is not atypical. It is particularly disconcerting and undoubtedly exacerbating that confusions about natural selection are common even among those responsible for teaching it Footnote 8 . As Nehm and Schonfeld ( 2007 ) recently concluded, “one cannot assume that biology teachers with extensive backgrounds in biology have an accurate working knowledge of evolution, natural selection, or the nature of science.”

Why is Natural Selection so Difficult to Understand?

Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population. While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers (Vlaardingerbroek and Roederer 1997 ; Rutledge and Mitchell 2002 ; Deniz et al. 2008 ), the two parameters seem to be at most only very weakly related in students Footnote 9 (Bishop and Anderson 1990 ; Demastes et al. 1995 ; Brem et al. 2003 ; Sinatra et al. 2003 ; Ingram and Nelson 2006 ; Shtulman 2006 ). Teachers notwithstanding, “it appears that a majority on both sides of the evolution-creation debate do not understand the process of natural selection or its role in evolution” (Bishop and Anderson 1990 ).

The second intuitive hypothesis is that most people simply lack formal education in biology and have learned incorrect versions of evolutionary mechanisms from non-authoritative sources (e.g., television, movies, parents). Inaccurate portrayals of evolutionary processes in the media, by teachers, and by scientists themselves surely exacerbate the situation (e.g., Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). However, this alone cannot provide a full explanation, because even direct instruction on natural selection tends to produce only modest improvements in students' understanding (e.g., Jensen and Finley 1995 ; Ferrari and Chi 1998 ; Nehm and Reilly 2007 ; Spindler and Doherty 2009 ). There also is evidence that levels of understanding do not differ greatly between science majors and non-science majors (Sundberg and Dini 1993 ). In the disquieting words of Ferrari and Chi ( 1998 ), “misconceptions about even the basic principles of Darwin's theory of evolution are extremely robust, even after years of education in biology.”

Misconceptions are well known to be common with many (perhaps most) aspects of science, including much simpler and more commonly encountered phenomena such as the physics of motion (e.g., McCloskey et al. 1980 ; Halloun and Hestenes 1985 ; Bloom and Weisberg 2007 ). The source of this larger problem seems to be a significant disconnect between the nature of the world as reflected in everyday experience and the one revealed by systematic scientific investigation (e.g., Shtulman 2006 ; Sinatra et al. 2008 ). Intuitive interpretations of the world, though sufficient for navigating daily life, are usually fundamentally at odds with scientific principles. If common sense were more than superficially accurate, scientific explanations would be less counterintuitive, but they also would be largely unnecessary.

Conceptual Frameworks Versus Spontaneous Constructions

It has been suggested by some authors that young students simply are incapable of understanding natural selection because they have not yet developed the formal reasoning abilities necessary to grasp it (Lawson and Thompson 1988 ). This could be taken to imply that natural selection should not be taught until later grades; however, those who have studied student understanding directly tend to disagree with any such suggestion (e.g., Clough and Wood-Robinson 1985 ; Settlage 1994 ). Overall, the issue does not seem to be a lack of logic (Greene 1990 ; Settlage 1994 ), but a combination of incorrect underlying premises about mechanisms and deep-seated cognitive biases that influence interpretations.

Many of the misconceptions that block an understanding of natural selection develop early in childhood as part of “naïve” but practical understandings of how the world is structured. These tend to persist unless replaced with more accurate and equally functional information. In this regard, some experts have argued that the goal of education should be to supplant existing conceptual frameworks with more accurate ones (see Sinatra et al. 2008 ). Under this view, “Helping people to understand evolution...is not a matter of adding on to their existing knowledge, but helping them to revise their previous models of the world to create an entirely new way of seeing” (Sinatra et al. 2008 ). Other authors suggest that students do not actually maintain coherent conceptual frameworks relating to complex phenomena, but instead construct explanations spontaneously using intuitions derived from everyday experience (see Southerland et al. 2001 ). Though less widely accepted, this latter view gains support from the observation that naïve evolutionary explanations given by non-experts may be tentative and inconsistent (Southerland et al. 2001 ) and may differ depending on the type of organisms being considered (Spiegel et al. 2006 ). In some cases, students may attempt a more complex explanation but resort to intuitive ideas when they encounter difficulty (Deadman and Kelly 1978 ). In either case, it is abundantly clear that simply describing the process of natural selection to students is ineffective and that it is imperative that misconceptions be confronted if they are to be corrected (e.g., Greene 1990 ; Scharmann 1990 ; Settlage 1994 ; Ferrari and Chi 1998 ; Alters and Nelson 2002 ; Passmore and Stewart 2002 ; Alters 2005 ; Nelson 2007 ).

A Catalog of Common Misconceptions

Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. It is clear from many studies that complex but accurate explanations of biological adaptation typically yield to naïve intuitions based on common experience (Fig.  2 ; Tables  2 and 3 ). As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common. All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students.

Teleology and the “Function Compunction”

Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs. Not surprisingly, human psychology includes a powerful bias toward thoughts about the “purpose” or “function” of objects and behaviors—what Kelemen and Rosset ( 2009 ) dub the “human function compunction.” This bias is particularly strong in children, who are apt to see most of the world in terms of purpose; for example, even suggesting that “rocks are pointy to keep animals from sitting on them” (Kelemen 1999a , b ; Kelemen and Rosset 2009 ). This tendency toward explanations based on purpose (“teleology”) runs very deep and persists throughout high school (Southerland et al. 2001 ) and even into postsecondary education (Kelemen and Rosset 2009 ). In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. It therefore reappears easily even in those with some basic scientific training; for example, in descriptions of ecological balance (“fungi grow in forests to help decomposition”) or species survival (“finches diversified in order to survive”; Kelemen and Rosset 2009 ).

Teleological explanations for biological features date back to Aristotle and remain very common in naïve interpretations of adaptation (e.g., Tamir and Zohar 1991 ; Pedersen and Halldén 1992 ; Southerland et al. 2001 ; Sinatra et al. 2008 ; Table  2 ). On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence (e.g., Bishop and Anderson 1990 ). On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need (Table  2 ). Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection (see Fig.  2 and Table  3 ).

Anthropomorphism and Intentionality

A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself (see below). In this sense, anthropomorphic misconceptions can be characterized as either internal (attributing adaptive change to the intentional actions of organisms) or external (conceiving of natural selection or “Nature” as a conscious agent; e.g., Kampourakis and Zogza 2008 ; Sinatra et al. 2008 ).

Internal anthropomorphism or “intentionality” is intimately tied to the misconception that individual organisms evolve in response to challenges imposed by the environment (rather than recognizing evolution as a population-level process). Gould ( 1980 ) described the obvious appeal of such intuitive notions as follows:

Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? Why not argue that organisms improve themselves by their own efforts and pass these advantages to their offspring in the form of altered genes—a process that has long been called, in technical parlance, the “inheritance of acquired characters.” This idea appeals to common sense not only for its simplicity but perhaps even more for its happy implication that evolution travels an inherently progressive path, propelled by the hard work of organisms themselves.

The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates (in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur), but also to plants and even to single-celled organisms. Thus, adaptations in any taxon may be described as “innovations,” “inventions,” or “solutions” (sometimes “ingenious” ones, no less). Even the evolution of antibiotic resistance is characterized as a process whereby bacteria “learn” to “outsmart” antibiotics with frustrating regularity. Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities (Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). Consider this particularly egregious example from a website maintained by the National Institutes of Health Footnote 10 :

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading—such as an antimicrobial—they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant.

Fundamentally inaccurate descriptions such as this are alarmingly common. As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language Footnote 11 . For example, this could read:

Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Use and Disuse

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin ( 1859 ), emphasizes changes to individual organisms that occur as they use particular features more or less. For example, Darwin ( 1859 ) invoked natural selection to explain the loss of sight in some subterranean rodents, but instead favored disuse alone as the explanation for loss of eyes in blind, cave-dwelling animals: “As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.” This sort of intuition remains common in naïve explanations for why unnecessary organs become vestigial or eventually disappear. Modern evolutionary theory recognizes several reasons that may account for the loss of complex features (e.g., Jeffery 2005 ; Espinasa and Espinasa 2008 ), some of which involve direct natural selection, but none of which is based simply on disuse.

Soft Inheritance

Evolution involving changes in individual organisms, whether based on conscious choice or use and disuse, would require that characteristics acquired during the lifetime of an individual be passed on to offspring Footnote 12 , a process often termed “soft inheritance.” The notion that acquired traits can be transmitted to offspring remained a common assumption among thinkers for more than 2,000 years, including into Darwin's time (Zirkle 1946 ). As is now understood, inheritance is actually “hard,” meaning that physical changes that occur during an organism's lifetime are not passed to offspring. This is because the cells that are involved in reproduction (the germline) are distinct from those that make up the rest of the body (the somatic line); only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur (though they could also arise very early in development and appear later in the adult offspring). Correct and incorrect interpretations of inheritance are contrasted in Fig.  3 .

A summary of correct ( left ) and incorrect ( right ) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of “hard inheritance”, whereas those on the right illustrate naïve mechanisms of “soft inheritance”. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black ) and those that make up the rest of the body (the somatic line, white ). In the top left panel , all cells in a parent organism initially contain a gene that specifies white coloration marked W ( A ). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G ( B ). This mutant gene is passed to the egg ( C ), which then develops into an offspring exhibiting gray coloration ( D ). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel , a parent once again begins with white coloration and the white gene in all of its cells ( H ). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions ( I ). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg ( J ), and the offspring exhibits none of the gray coloration that was acquired by its parent ( K ). In the top right panel , the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration ( P ) changes during its lifetime to become gray ( Q ). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg ( R ), and the offspring is born with the gray color acquired by its parent ( S ). In the bottom right panel , a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells ( W ). A mutation occurs in one type of body cells to change those cells to gray ( X ). A mixture of white and gray genes is passed on to the egg ( Y ), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent ( Z ). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig.  2 )

Studies have indicated that belief in soft inheritance arises early in youth as part of a naïve model of heredity (e.g., Deadman and Kelly 1978 ; Kargbo et al. 1980 ; Lawson and Thompson 1988 ; Wood-Robinson 1994 ). That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Nature as a Selecting Agent

Thirty years ago, widely respected broadcaster Sir David Attenborough ( 1979 ) aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:

Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way–that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough ( 1979 ) admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. In it, natural selection is described as a “driving force” that rigorously “sifts” genetic variation, which could be misunderstood to imply that it takes an active role in prompting evolutionary change. Much more seriously, one often encounters descriptions of natural selection as a processes that “chooses” among “preferred” variants or “experiments with” or “explores” different options. Some expressions, such as “favored” and “selected for” are used commonly as shorthand in evolutionary biology and are not meant to impart consciousness to natural selection; however, these too may be misinterpreted in the vernacular sense by non-experts and must be clarified.

Darwin ( 1859 ) himself could not resist slipping into the language of agency at times:

It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Perhaps recognizing the ease with which such language can be misconstrued, Darwin ( 1868 ) later wrote that “The term ‘Natural Selection’ is in some respects a bad one, as it seems to imply conscious choice; but this will be disregarded after a little familiarity.” Unfortunately, more than “a little familiarity” seems necessary to abandon the notion of Nature as an active decision maker.

Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth ( 1975a , b , 1977 ) bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students (see also Bishop and Anderson 1990 ; Alters and Nelson 2002 ; Moore et al. 2002 ; Sinatra et al. 2008 ). That said, a study of high school students by Tamir and Zohar ( 1991 ) suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation (i.e., merely a convenient description) versus an anthropomorphic/teleological explanation (i.e., involving conscious intent or goal-oriented mechanisms as causal factors; see also Bartov 1978 , 1981 ). Moore et al. ( 2002 ), by contrast, concluded from their study of undergraduates that “students fail to distinguish between the relatively concrete register of genetics and the more figurative language of the specialist shorthand needed to condense the long view of evolutionary processes” (see also Jungwirth 1975a , 1977 ). Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction (Zohar and Ginossar 1998 ). In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Source Versus Sorting of Variation

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next. This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation (e.g., Greene 1990 ; Creedy 1993 ; Moore et al. 2002 ). Whereas an accurate understanding recognizes that most new mutations are neutral or harmful in a given environment, such naïve interpretations assume that mutations occur as a response to environmental challenges and therefore are always beneficial (Fig.  2 ). For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter Footnote 13 . Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants. Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination.

Typological, Essentialist, and Transformationist Thinking

Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Since Darwin ( 1859 ), evolutionary theory has been based strongly on “population” thinking that emphasizes differences among individuals. By contrast, many naïve interpretations of evolution remain rooted in the “typological” or “essentialist” thinking that has existed since the ancient Greeks (Mayr 1982 , 2001 ; Sinatra et al. 2008 ). In this case, species are conceived of as exhibiting a single “type” or a common “essence,” with variation among individuals representing anomalous and largely unimportant deviations from the type or essence. As Shtulman ( 2006 ) notes, “human beings tend to essentialize biological kinds and essentialism is incompatible with natural selection.” As with many other conceptual biases, the tendency to essentialize seems to arise early in childhood and remains the default for most individuals (Strevens 2000 ; Gelman 2004 ; Evans et al. 2005 ; Shtulman 2006 ).

The incorrect belief that species are uniform leads to “transformationist” views of adaptation in which an entire population transforms as a whole as it adapts (Alters 2005 ; Shtulman 2006 ; Bardapurkar 2008 ). This contrasts with the correct, “variational” understanding of natural selection in which it is the proportion of traits within populations that changes (Fig.  2 ). Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges. Indeed, Shtulman ( 2006 ) found that transformationists appeal to “need” as a cause of evolutionary change three times more often than do variationists.

Events and Absolutes Versus Processes and Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

Natural selection is mistakenly seen as an event rather than as a process (Ferrari and Chi 1998 ; Sinatra et al. 2008 ). Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented. By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented (Ferrari and Chi 1998 ). Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. Viewing natural selection as a single event can also lead to incorrect “saltationist” assumptions in which complex adaptive features are imagined to appear suddenly in a single generation (see Gregory 2008b for an overview of the evolution of complex organs).

Natural selection is incorrectly conceived as being “all or nothing,” with all unfit individuals dying and all fit individuals surviving. In actuality, it is a probabilistic process in which some traits make it more likely—but do not guarantee—that organisms possessing them will successfully reproduce. Moreover, the statistical nature of the process is such that even a small difference in reproductive success (say, 1%) is enough to produce a gradual increase in the frequency of a trait over many generations.

Concluding Remarks

Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Though it is based on well-established and individually straightforward components, a proper grasp of the mechanism and its implications remains very rare among non-specialists. The unavoidable conclusion is that the vast majority of individuals, including most with postsecondary education in science, lack a basic understanding of how adaptive evolution occurs.

While no concrete solutions to this problem have yet been found, it is evident that simply outlining the various components of natural selection rarely imparts an understanding of the process to students. Various alternative teaching strategies and activities have been suggested, and some do help to improve the level of understanding among students (e.g., Bishop and Anderson 1986 ; Jensen and Finley 1995 , 1996 ; Firenze 1997 ; Passmore and Stewart 2002 ; Sundberg 2003 ; Alters 2005 ; Scharmann 1990 ; Wilson 2005 ; Nelson 2007 , 2008 ; Pennock 2007 ; Kampourakis and Zogza 2008 ). Efforts to integrate evolution throughout biology curricula rather than segregating it into a single unit may also prove more effective (Nehm et al. 2009 ), as may steps taken to make evolution relevant to everyday concerns (e.g., Hillis 2007 ).

At the very least, it is abundantly clear that teaching and learning natural selection must include efforts to identify, confront, and supplant misconceptions. Most of these derive from deeply held conceptual biases that may have been present since childhood. Natural selection, like most complex scientific theories, runs counter to common experience and therefore competes—usually unsuccessfully—with intuitive ideas about inheritance, variation, function, intentionality, and probability. The tendency, both outside and within academic settings, to use inaccurate language to describe evolutionary phenomena probably serves to reinforce these problems.

Natural selection is a central component of modern evolutionary theory, which in turn is the unifying theme of all biology. Without a grasp of this process and its consequences, it is simply impossible to understand, even in basic terms, how and why life has become so marvelously diverse. The enormous challenge faced by biologists and educators in correcting the widespread misunderstanding of natural selection is matched only by the importance of the task.

For a more advanced treatment, see Bell ( 1997 , 2008 ) or consult any of the major undergraduate-level evolutionary biology or population genetics textbooks.

The Origin was, in Darwin's words, an “abstract” of a much larger work he had initially intended to write. Much of the additional material is available in Darwin ( 1868 ) and Stauffer ( 1975 ).

See Gregory ( 2008a ) for a discussion regarding the use of the term “theory” in science.

Ridley ( 2004 ) points out that Darwin's calculations require overlapping generations to reach this exact number, but the point remains that even in slow-reproducing species the rate of potential production is enormous relative to actual numbers of organisms.

Humans are currently undergoing a rapid population expansion, but this is the exception rather than the rule. As Darwin ( 1859 ) noted, “Although some species may now be increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.”

It cannot be overemphasized that “evolution” and “natural selection” are not interchangeable. This is because not all evolution occurs by natural selection and because not all outcomes of natural selection involve changes in the genetic makeup of populations. A detailed discussion of the different types of selection is beyond the scope of this article, but it can be pointed out that the effect of “stabilizing selection” is to prevent directional change in populations.

Instructors interested in assessing their own students' level of understanding may wish to consult tests developed by Bishop and Anderson ( 1986 ), Anderson et al. ( 2002 ), Beardsley ( 2004 ), Shtulman ( 2006 ), or Kampourakis and Zogza ( 2009 ).

Even more alarming is a recent indication that one in six teachers in the USA is a young Earth creationist, and that about one in eight teaches creationism as though it were a valid alternative to evolutionary science (Berkman et al. 2008 ).

Strictly speaking, it is not necessary to understand how evolution occurs to be convinced that it has occurred because the historical fact of evolution is supported by many convergent lines of evidence that are independent of discussions about particular mechanisms. Again, this represents the important distinction between evolution as fact and theory. See Gregory ( 2008a ).

http://www3.niaid.nih.gov/topics/antimicrobialResistance/Understanding/history.htm , accessed February 2009.

One should always be wary of the linguistic symptoms of anthropomorphic misconceptions, which usually include phrasing like “so that” (versus “because”) or “in order to” (versus “happened to”) when explaining adaptations (Kampourakis and Zogza 2009 ).

It must be noted that the persistent tendency to label the inheritance of acquired characteristics as “Lamarckian” is false: Soft inheritance was commonly accepted long before Lamarck's time (Zirkle 1946 ). Likewise, mechanisms involving organisms' conscious desires to change are often incorrectly attributed to Lamarck. For recent critiques of the tendency to describe various misconceptions as Lamarckian, see Geraedts and Boersma ( 2006 ) and Kampourakis and Zogza ( 2007 ). It is unfortunate that these mistakenly attributed concepts serve as the primary legacy of Lamarck, who in actuality made several important contributions to biology (a term first used by Lamarck), including greatly advancing the classification of invertebrates (another term he coined) and, of course, developing the first (albeit ultimately incorrect) mechanistic theory of evolution. For discussions of Lamarck's views and contributions to evolutionary biology, see Packard ( 1901 ), Burkhardt ( 1972 , 1995 ), Corsi ( 1988 ), Humphreys ( 1995 , 1996 ), and Kampourakis and Zogza ( 2007 ). Lamarck's works are available online at http://www.lamarck.cnrs.fr/index.php?lang=en .

One may wonder how this misconception is reconciled with the common admonition by medical doctors to complete each course of treatment with antibiotics even after symptoms disappear—would this not provide more opportunities for bacteria to “develop” resistance by prolonging exposure?

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Gregory, T.R. Understanding Natural Selection: Essential Concepts and Common Misconceptions. Evo Edu Outreach 2 , 156–175 (2009). https://doi.org/10.1007/s12052-009-0128-1

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Survival of the fittest

Natural selection was the term Darwin used to describe the evolutionary process by which favorable or advantageous traits and characteristics are preserved and unfavorable or disadvantageous ones discarded.

Introduction

Natural selection was the term Charles Darwin (1809–1882) used for the main mechanism by which he understood evolution to work. Natural selection was first announced publicly in a joint reading of his and Alfred Russel Wallace’s papers at the Linnean Society in July 1858 (Darwin and Wallace 1858 ) and first developed in published form in Darwin’s most important book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life ( 1859 ). As the full title of the Origin indicates, natural selection was key to Darwin’s evolutionary argument, and the fourth chapter of the Origin was devoted to an exposition of its operation. Darwin’s definition of natural selection was...

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Stack, D. (2019). Charles Darwin: Theory of Natural Selection. In: Shackelford, T., Weekes-Shackelford, V. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-16999-6_1382-1

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Natural Selection Essay Guide - Charles Darwin's Evolution Theory Essay

In a natural selection essay, students should observe the famous theory proposed by Charles Darwin, who insisted that the man is descended from an ape. The concept was rejected later, but it is still very popular in academia. That is why you should be ready to write various assignments focused on the theory of evolution suggested by a scientist.

No matter whether you agree or disagree with Charles Darwin, you can express your ideas and analyze the theory from different perspectives by natural selection essay. That is an assignment you can be required to write for your biology, anthropology, geography, or history class - biology professors are ones interested in this theory the most. But first, let's decide on what the features, standards, and purpose of this writing are.

What is Natural Selection Essay?

Let's discuss what a natural selection is. It is the varying ways of survival and reproduction of living creatures that depend on different phenotypes. Here, we can see that an essay on such a specific topic will contain many professional and scientific terms that you'll have to explain. You don't have to turn your assignment into a dictionary. But, don't forget that an essay should be coherent for your target audience.

As for the academic paper on this topic, it has to discuss whether a natural selection is one of the fundamental elements of evolution. Such essays may also cover the discussion about other issues such as mutation, migration, and genes.

Darwin's theory is quite easy, but it can be explained in different ways, so you have to obtain a stance on your attitude toward this concept. That will be your thesis statement, and the task would be to support it well with reliable facts and appropriate examples.

The Process of Natural Selection Essay Writing

Here are some basic concepts you can cover in your Charles Darwin natural selection assignment:

• A variation in characteristics exist. For instance, some people have white skin, while others may have red or black. The same can be said about the eye shapes, skin thickness, etc.
• There is a difference in reproduction. The surrounding environment doesn't allow the endless expansion of the population. Thus, some creatures can reproduce to their full potential, while others cannot do that. For instance, small fishes are eaten by various types of birds and predators. Therefore, such fish species survive by reproducing more frequently than others.
• Heredity matters. First, you should admit that there is such thing is heredity. Genes play an important role in our development. For example, men who used to have male parents with problematic hair tend to go bald more frequently than those who didn't. Also, here, you can cover such a phenomenon as selective breeding that explains how people develop particular features in different species of animals and plants.

If you mention some of those issues in your essay on natural selection, you'll have a better chance to get a higher grade. We recommend basing your assignment on the rules suggested by Darwin. That will show your competence in a matter and ability to think analytically.

Now, look at this academic paper under the microscope. Below, you can find an outline for your natural selection essay.

Natural Selection Essay Outline Example

Most essays have a standard structure, but you may expand it depending on your topic and objectives. You may analyze various essay prompts on evolution mechanisms before developing an outline. This way, you'll understand what structure will be winning for your writing, what paragraphs to include, and how to preserve the logic of your text.

Having an outline will help to follow the logical flow of thoughts. It serves as a map or compass in the woods - no traveler will be able to survive without having these critical items. An outline will prevent you from facing a writer's block, so prepare it no matter whether your instructor requires you to do that. Just use it as a draft while writing your natural selection essay.

Introduction

The simplest way is to start with the general info - a natural selection definition. You may admit that the theory and term were first introduced by Charles Darwin. Mention that he tried to prove that differentiation is present within all organisms - it's the major element of evolution. You can also analyze what people knew about evolution before Darwin's discovery. End the opening paragraph with a powerful thesis explaining your stance. It could be as simple and naive as: "I support the evolution theory offered by Charles Darwin even though it was denied. I cannot ignore the similarities between human beings and monkeys, and I am going to prove at least major statements of the famous scientist in this essay."

Body paragraphs

You should come up with three arguments minimum to defend your thesis and structure them into three body paragraphs. Each of them should open with a topic sentence related to your thesis. The rest of the paragraph should contain some evidence gathered from credible external sources and your own thoughts on an issue along with examples. Include transition words like to begin with , unlike , in contrast , and others to show the link between different parts of your essay.

Open the final paragraph with the restated thesis and provide a summary of all topic sentences. But, that is quite a worn-out approach. You can sum everything up by explaining the influence of Darwin's theories on today's science or offering your own perception of the phenomenon's importance. Sometimes, if the essay is about an arguable issue, students finish it with an intriguing hook. For instance, you can provoke the readers to debate over the study or make them answer certain questions.

Do not forget to add a bibliography list at the end of your work. Mention all the scientific sources you used while working on your project.

10 Natural Selection and Evolution Essay Prompts

Don't know how to formulate an interesting and relevant topic for an essay of this type? We offer you a list of prompts that convey different aspects of the natural selection concept.

• The process of natural selection.
• Sexual selection theory.
• Cultural evolution from the aspect of Charles Darwin.
• Reconciling Darwin's famous theory of evolution.
• Modern evolutionary synthesis of the theories offered by Darwin.
• The concept of Social Darwinism in the development of people's relationships.
• The main concepts of evolution through natural selection.
• Summary of Charles Darwin's essay on natural selection.
• Discovery of natural selection in different species - specifics and common features.
• Darwin evolution in relation to Intelligent design.

That information is enough to craft a good paper on natural selection or evolution. In case of any questions, you may use our essay writing service and order a top-notch assignment in two clicks.

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